in solution can be large. Interaction of
silica-rich brines with flood runoff may
cause relatively sudden supersaturation
with respect to amorphoussilica, and
thereby lead to inorganic precipitation
of chert. If there is rapid mixing of
runoff with brine, much silica may remain in the diluted waters. If a strati-
fied lake forms, however, biogenic CO,
may be retained in the hypolimnion and
reduce the pH of the bottom brines; in
this manner the bulk of the dissolved
silica can be precipitated. Figure 1
shows that a drop in pH from 11.0 to
8.5 can cause precipitation of as much
as 3000 ppm SiO., which corresponds
to a 1.5-mm-thick layer of chert for
each meter of depth of brine. Silica
layers that were probably formed by
this mechanism have been found in the
High Magadi beds, of Pleistocene age,
and within the Alkali Valley playa
deposits (77).
BLAIR F. JoNEs
SHIRLEY L, RETTIG
U.S. Geological Survey,
Washington, D.C, 20242
Hans P. EuGSTER
Departinent of Geology,
Johns Hopkins University,
References and Notes
1.
S. N. Davis, Amer. J, Sci. 262, 870 (1964);
D. Livingston, U.S. Geol. Surv. Profess.
2.
G, W. Morey, R. O. Fournier, J. J. Rowe,
J, Geophys. Res. 69, 1995 (1964); R. Siever,
4.
5.
6.
Indicators of Pleistocene Glaciation
' Abstract. Selected species of Coccolithaphoridae from recent sediments and midWisconsin glacial sediments of the North Atlantic were examined in an attempt
to determine cooling effects. All species showed a definite shift southward during
the glacial period. The average shift in this planktonic population was 15 degrees
of latitude, with the greatest change in the eastern Atlantic. A paleoisotherm map
can be drawn on the basis of the temperature boundaries of coccolithophorids. The
species boundaries indicate a possible shift in position of the subtropical gyral
to a glacial position roughly parallel to the 33-degree line of latitude.
The dramatic fluctuations in Pleistocene climate are recorded in sediments
in the Atlantic Ocean (/), but unfortu-
nately the means of procuring these
data are poorly developed. The only direct technique available at the present
time is the use of oxygen isotopes (2).
This report deals with a new approach
—plotting the migration of biogeographic boundaries for temperaturerestricted species of Coccolithophoridae
due to Pleistocene glaciation.
Among all the microorganisms that
leave fossil records in oceanic sediments, the Coccolithophoridae probably
have the greatest potential as paleoclimatic indicators. In addition to their
wide geographic distribution and stable
mineral skeleton (calcite), these marine
Baltimore, Maryland 21218
3.
Coccoliths as Paleoclimatic
Paper 440 (1963).
algae inhabit the upper euphotic zone
(3-5) and consequently are under direct climatic control. In living species
80°
J, Geol, 1962, 127 (1962).
G. J. §, Govett, Bull, Geol, Soc. Amer, 77,
1191 (1966); M. N, A. Peterson and C. C.
Von der Bosch, Science 149, 1501 (1965).
G. J. §. Govett, Anal. Chem, Acta 25, 69
(1961).
Ringbom,
Ahlers,
Siitonen,
ibid.
20,
70°
60°
50°
it is possible to correlate biogeographic
boundaries with surface water isotherms
(4), and this is the basis of my report.
The method of attack, being biogeo-
graphic, requires the widest possible
geographical distribution of core material. This is not easily obtained, for,
although the North Atlantic has been
the site of intensive sampling, there remain large gaps in the core distribution. A limiting factor is that large
areas of the North Atlantic basin are
below the carbonate compensation level,
with a consequent lack of coccolith
flora. Thus the 23 cores sampled
(Table 1) are restricted to three linear
belts. Two cover the shelf, slope, and
rise of both North America and Eu-
rope-Africa; the third, the Mid-Atlantic
Ridge.
Choice of the particular species to be
examined requires that two separate cri40°
=—30°
20°
|O°
78
(1959).
A. §. VanDenburgh, Geol. Soc. Amer. Spec.
Paper 82 (1964), p. 349.
7. I. 8. Allison and R. S. Mason, Oregon Dept.
Geol. Mineral Ind. Short Paper 17 (1947).
8 S. L. Rettig and B. F. Jones, U.S. Geol.
Surv. Profess. Paper 501-D (1964), p. 134.
>. B. F. Jones, U.S. Geol. Surv. Profess. Paper
502-A (1965), 56 pp.
10. B. H. Baker, Geol. Surv. Kenya Rept. 42
11.
12.
13.
14,
15.
16.
17.
18.
50°
(1958).
G. Lagerstrom, Acta Chem. Seand. 13, 722
(1989).
N. Ingri, ibid., p. 758.
J. H. Feth, C. E. Roberson, S. M. Rogers,
Geochim. Cosmochim, Acta 22, 75 (1961).
R. Wollast, ibid. 31, 635 (1967}.
J. A. McKeague and M. G. Cline, Can.
J. Soil Sci, 43, 70 (1963).
B. F. Jones, Northern Ohio Geol. Soc. Symp.
Sait 2nd (Cleveland, 1966}, p. 181.
H. P. Eugster, B. F. Jones, R. A, Shepard,
unpublished abstract, Geol. Soc, Amer., 1967;
H. P. Eugster, unpublished.
Authorized by the director, U.S. Geological
Survey. Aided by a grant from the Petroleum
Research Fund of Amer, Chem. Soc. We
thank A. H. Truesdell and A. 8. Van Denbureh for discussion and field assistance in
Oregon, O. P. Bricker and R. O. Fournier
for comments on the manuscript, and M. D.
Edwards for aid with the regression analyses.
We also thank the Magadi Soda Co. of
Kenya for its cooperation.
tw
2 August 1967
1314
ae?”
ove
20°
Coccolithus petagicus — --—.——
Umbeilosphaera irreguiaris -++-+-+++-«“
Helicosphcera corteri
1O? —|
Rhabdosphaera stylifera — — —
IO°
Syracosphaera puicha + 4+- —- +
R= Recent
O°
G= Glacial
Te
soe
70°
60°
O°
1
30°
ee
wee ee
ee
eee
ee
QO
20°~—s«*I'0?
Fig. 1. Species population boundaries for Recent and mid-Wisconsin time.
SCIENCE, VOL, 158